Still fairly useless for VHF/UHF. If you put a full size UHF dipole in front of a DirecTV Slimline dish it would simply be a 2 element Yagi with a large cumbersome reflector. You might get 6dBi or 3.86dBd gain at UHF at the most. And you would not be putting that at the focal point, it would be closer to 1/4 wavelength out as there is no useable "focal point" at UHF or VHF. The reflector is not even big enough to use as a reflector on VHF or NOAA weather freqs, so that would be a waste of time. Stick a UHF rubber duck in front of one and you would sort of have a 2 element Yagi but with reduced gain over a dipole feed, so basically useless.
For Ku and Ka band its a completely different story where the LNB feed horns illuminate the reflector and are designed to be about 10dB down at the edges of the dish to avoid too much spillover. There is an actual focal point where a good amount of the parabolic shape focuses into the feed horn and if my memory is still intact the gain at Ku band (12.2 to 12.7GHz) is around 34dBi. I forget what the Ka band gain is but I have it somewhere.
The DirecTV Slimline dish is also designed to see 5 orbital slots spanning about 20 degrees apart so the dish is a modified parabola to make all that work. None of this is any good for use at VHF/UHF and if anything it would further degrade use at VHF/UHF.
Useless for VHF UHF? If he mounts a rubber duck at the focal point it should be good for directional but heavier more wind loading and more difficult to aim as a yagi cut for resonance. Yes, the LNB's pictured are not "terrestrialy" friendly for anything but if the dish itself reflects Ku and Ka it will certainly work for VHF and UHF. A easy test try NOAA.